for National Geographic News
The recent discovery of carbon dioxide in the atmosphere of a Jupiter-like planet 63 light-years away has some researchers excited that we may soon find habitable exoplanets—worlds circling other stars.
According to lead author Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, California, carbon dioxide is a biomarker, a molecule associated with life as we know it.
This first discovery of the molecule on a far-flung planet, he said, is a step toward eventually finding biomarkers on smaller, more Earthlike worlds.
Last March, Swain and colleagues had announced the first detection of another biologically important chemical, methane, on the same exoplanet, as well as confirmation of water vapor.
"Methane is a potential marker [of life], as is water, as is carbon dioxide," Swain said. "So, three of the biggies we've already detected."
But not all planetary scientists are embracing the idea that finding chemicals such as carbon dioxide and methane on a distant planet is enough to say it is habitable.
On Earth, for example, carbon dioxide traps a certain amount of sunlight, keeping temperatures warm enough to support life. But on Venus, too much carbon dioxide creates a killing heat—average temperatures there reach 864 degrees Fahrenheit (462 degrees Celsius).
"To me, a CO2-dominated atmosphere says the planet would certainly be more like Venus than Earth," said Ellen Stofan, a planetary geologist for Proxemy Research in Maryland and a member of NASA's Cassini science team.
"To say it precludes life or guarantees life are both going out on a limb—we just don't know. But it is intriguing!"
Swain and colleagues made the CO2 discovery while studying radiation from an exoplanet dubbed HD 189733b.
The planet periodically transits—or passes in front of—its host star, giving observers a measurement of the star and planet's combined light.
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